1.1 Applications and services of satellite networks

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1 1 Introduction This chapter aims to introduce the basic concepts of satellite networking including applications and services, circuit and packet switching, broadband networks, network protocols and reference models, characteristics of satellite networks, internetworking between satellite and terrestrial networks and convergence of network technologies and protocols. When you have completed this chapter, you should be able to: Understand the concepts of satellite networks and internetworking with terrestrial networks. Know the different satellite services, network services and quality of service (QoS). Appreciate the differences between satellite networking and terrestrial networking issues. Describe the functions of network user terminals and satellite user earth terminals and gateway earth terminals. Know the basic principles of protocols and the ISO reference model. Know the basic ATM reference model. Know the basic Internet TCP/IP protocol suite. Understand the basic concepts of multiplexing and multiple accessing. Understand the basic switching concepts including circuit switching, virtual circuit switching and routeing. Understand the evolution process and convergence of network technologies and protocols. 1.1 Applications and services of satellite networks Satellites are manmade stars in the sky, and are often mistaken for real stars. To many people, they are full of mystery. Scientists and engineers love to give life to them by calling them birds like birds, they fly where other creatures can only dream. They watch the earth from the sky, help us to find our way around the world, carry our telephone calls, s Satellite Networking: Principles and Protocols 2005 John Wiley & Sons, Ltd Zhili Sun

2 2 Satellite Networking: Principles and Protocols TV Inter-satellite link (ISL) Fixed earth station Handheld terminal Terrestrial Network User terminals: telephone, fax, computer Transportable earth stations Portable earth station Terrestrial Network Figure 1.1 Typical applications and services of satellite networking and web pages, and relay TV programmes across the sky. Actually the altitudes of satellites are far beyond the reach of any real bird. When satellites are used for networking, their high altitude enables them to play a unique role in the global network infrastructure (GNI). Satellite networking is an expanding field, which has developed significantly since the birth of the first telecommunication satellite, from traditional telephony and TV broadcast services to modern broadband and Internet networks and digital satellite broadcasts. Many of the technological advances in networking areas are centred on satellite networking. With increasing bandwidth and mobility demands in the horizon, satellite is a logical option to provide greater bandwidth with global coverage beyond the reach of terrestrial networks, and shows great promise for the future. With the development of networking technologies, satellite networks are becoming more and more integrated into the GNI. Therefore, internetworking with terrestrial networks and protocols is an important part of satellite networking. The ultimate goal of satellite networking is to provide services and applications. User terminals provide services and applications directly to users. The network provides transportation services to carry information between users for a certain distance. Figure 1.1 illustrates a typical satellite network configuration consisting of terrestrial networks, satellites with an inter-satellite link (ISL), fixed earth stations, transportable earth stations, portable and handheld terminals, and user terminals connecting to satellite links directly or through terrestrial networks Roles of satellite networks In terrestrial networks, many links and nodes are needed to reach long distances and cover wide areas. They are organised to achieve economical maintenance and operation of the networks. The nature of satellites makes them fundamentally different from terrestrial net-

3 Introduction 3 works in terms of distances, shared bandwidth resources, transmission technologies, design, development and operation, and costs and needs of users. Functionally, satellite networks can provide direct connections among user terminals, connections for terminals to access terrestrial networks, and connections between terrestrial networks. The user terminals provide services and applications to people, which are often independent from satellite networks, i.e. the same terminal can be used to access satellite networks as well as terrestrial networks. The satellite terminals, also called earth stations, and are the earth segment of the satellite networks, providing access points to the satellite networks for user terminals via the user earth station (UES) and for terrestrial networks via the gateway earth station (GES). The satellite is the core of satellite networks and also the centre of the networks in terms of both functions and physical connections. Figure 1.2 illustrates the relationship between user terminal, terrestrial network and satellite network. Typically, satellite networks consist of satellites interconnecting a few large GES and many small UES. The small GES are used for direct access by user terminals and the large UES for connecting terrestrial networks. The satellite UES and GES define the boundary of the satellite network. Like other types of networks, users access satellite networks through the boundary. For mobile and transportable terminals, the functions of user terminal and satellite UES are integrated into a single unit, but for transportable terminals their antennas are distinguishably visible. The most important roles of satellite networks are to provide access by user terminals and to internetwork with terrestrial networks so that the applications and services provided by terrestrial networks such as telephony, television, broadband access and Internet connections can be extended to places where cable and terrestrial radio cannot economically be installed and maintained. In addition, satellite networks can also bring these services and applications to ships, aircraft, vehicles, space and places beyond the reach of terrestrial networks. Satellites also play important roles in military, meteorology, global positioning systems (GPS), observation of environments, private data and communication services, and future development of new services and applications for immediate global coverage such as User terminal Application software Network software Network hardware Network software Network hardware Terrestrial network Network access points Satellite User Earth Station (UES) Network software Network hardware Satellite Gateway Earth Station (GES) Network software Network hardware Thin route link Thick pipe link Satellite Inter Satellite Link (ISL) Figure 1.2 Functional relationships of user terminal, terrestrial network and satellite network

4 4 Satellite Networking: Principles and Protocols broadband network, and new generations of mobile networks and digital broadcast services worldwide Network software and hardware In terms of implementation, the user terminal consists of network hardware and software and application software. The network software and hardware provide functions and mechanisms to send information in correct formats and to use the correct protocols at an appropriate network access point. They also receive information from the access point. Network hardware provides signal transmission making efficient and cost-effective use of bandwidth resources and transmission technologies. Naturally, a radio link is used to ease mobility of the user terminals associated with access links; and high-capacity optical fibre is used for backbone connections. With the advance of digital signal processing (DSP), traditional hardware implementations are being replaced more and more by software to increase the flexibility of reconfiguration, hence reducing costs. Therefore the proportion of implementation becomes more and more in software and less and less in hardware. Many hardware implementations are first implemented and emulated in software, though hardware is the foundation of any system implementation. For example, traditional telephone networks are mainly in hardware; and modern telephone networks, computer and data networks and the Internet are mainly in software Satellite network interfaces Typically, satellite networks have two types of external interfaces: one is between the satellite UES and user terminals; and the other is between the satellite GES and terrestrial networks. Internally, there are three types of interfaces: between the UES and satellite communication payload system; between the GES and satellite communication payload system; and the inter-satellite link (ISL) between satellites. All use radio links, except that the ISL may also use optical links. Like physical cables, radio bandwidth is one of the most important and scarce resources for information delivery over satellite networks. Unlike cables, bandwidth cannot be manufactured, it can only be shared and its use maximised. The other important resource is transmission power. In particular, power is limited for user terminals requiring mobility or for those installed in remote places that rely on battery supply of power, and also for communication systems on board satellites that rely on battery and solar energy. The bandwidth and transmission power together within the transmission conditions and environment determine the capacity of the satellite networks. Satellite networking shares many basic concepts with general networking. In terms of topology, it can be configured into star or mesh topologies. In terms of transmission technology, it can be set up for point-to-point, point-to-multipoint and multipoint-to-multipoint connections. In terms of interface, we can easily map the satellite network in general network terms such as user network interface (UNI) and network nodes interface (NNI). When two networks need to be connected together, a network-to-network interface is needed, which is the interface of a network node in one network with a network node in

5 Introduction 5 another network. They have similar functions as NNI. Therefore, NNI may also be used to denote a network-to-network interface Network services The UES and GES provide network services. In traditional networks, such services are classified into two categories: teleservices and bearer services. The teleservices are highlevel services that can be used by users directly such as telephone, fax service, video and data services. Quality of service (QoS) at this level is user centric, i.e. the QoS indicates users perceived quality, such as mean objective score (MOS). The bearer services are lower level services provided by the networks to support the teleservices. QoS at this level is network centric, i.e. transmission delay, delay jitter, transmission errors and transmission speed. There are methods to map between these two levels of services. The network needs to allocate resources to meet the QoS requirement and to optimise the network performance. Network QoS and user QoS have contradicting objectives adjustable by traffic loads, i.e. we can increase QoS by reducing traffic load on the network or by increasing network resources, however, this may decrease the network utilisation for network operators. Network operators can also increase network utilisation by increasing traffic load, but this may affect user QoS. It is the art of traffic engineering to optimise network utilisation with a given network load under the condition of meeting user QoS requirements Applications Applications are combinations of one or more network services. For example, tele-education and telemedicine applications are based on combinations of voice, video and data services. Combinations of voice, video and data are also called multimedia services. Some applications can be used with the network services to create new applications. Services are basic components provided by the network. Applications are built from these basic components. Often the terms application and service are used interchangeably in the literature. Sometimes it is useful to distinguish them. 1.2 ITU-R definitions of satellite services Satellite applications are based on the basic satellite services. Due to the nature of radio communications, the satellite services are limited by the available radio frequency bands. Various satellite services have been defined, including fixed satellite service (FSS), mobile satellite service (MSS) and broadcasting satellite service (BSS) by the ITU Radiocommunication Standardisation Sector (ITU-R) for the purpose of bandwidth allocation, planning and management Fixed satellite service (FSS) The FSS is defined as a radio communication service between a given position on the earth s surface when one or more satellites are used. These stations at the earth surface are called earth stations of FSS. Stations located on board satellites, mainly consisting of

6 6 Satellite Networking: Principles and Protocols the satellite transponders and associated antennas, are called space stations of the FSS. Of course, new-generation satellites have onboard sophisticated communication systems including onboard switching. Communications between earth stations are through one satellite or more satellites interconnected through ISL. It is also possible to have two satellites interconnected through a common earth station without an ISL. FSS also includes feeder links such as the link between a fixed earth station and satellite for broadcasting satellite service (BSS) and mobile satellite service (MSS). The FSS supports all types of telecommunication and data network services such as telephony, fax, data, video, TV, Internet and radio Mobile satellite service (MSS) The MSS is defined as a radio communication service between mobile earth stations and one or more satellites. This includes maritime, aeronautical and land MSS. Due to mobility requirements, mobile earth terminals are often small, and some are even handheld terminals Broadcasting satellite service (BSS) The BSS is a radio communication service in which signals transmitted or retransmitted by satellites are intended for direct reception by the general public using a TV receiving only antenna (TVRO). The satellites implemented for the BSS are often called direct broadcast satellites (DBS). The direct receptions include individual direct to home (DTH) and community antenna television (CATV). The new generation of BSS may also have a return link via satellite Other satellite services Some other satellite services are designed for specific applications such as military, radio determination, navigation, meteorology, earth surveys and space exploration. A set of space stations and earth stations working together to provide radio communication is called a satellite system. For convenience, sometimes the satellite system or a part of it is called a satellite network. We will see in the context of network protocols that the satellite system may not need to support all the layers of functions of the protocol stack (physical layer, link layer or network layer). 1.3 ITU-T definitions of network services During the process of developing broadband communication network standards, the ITU Telecommunication Standardisation Sector (ITU-T) has defined telecommunication services provided to users by networks. There are two main classes of services: interactive and distribution services, which are further divided into subclasses Interactive services Interactive services offer one user the possibility to interact with another user in real-time conversation and messages or to interact with information servers in computers. It can

7 Introduction 7 be seen that different services may have different QoS and bandwidth requirements from the network to support these services. The subclasses of the interactive services are defined as the following: Conversational services: conversational services in general provide the means for bidirectional communication with real-time (no store-and-forward) end-to-end information transfer from user to user or between user and host (e.g. for data processing). The flow of the user information may be bidirectional symmetric, bidirectional asymmetric and in some specific cases (e.g. such as video surveillance), the flow of information may be unidirectional. The information is generated by the sending user or users, and is dedicated to one or more of the communication partners at the receiving site. Examples of broadband conversational services are telephony, videotelephony, and videoconference. Messaging services: messaging services offer user-to-user communication between individual users via storage units with store-and-forward, mailbox and/or message handling (e.g. information editing, processing and conversion) functions. Examples of broadband messaging services are message-handling services and mail services for moving pictures (films), high-resolution images and audio information. Retrieval services: the user of retrieval services can retrieve information stored in information centres provided for public use. This information will be sent to the user by demand only. The information can be retrieved on an individual basis. Moreover, the time at which an information sequence starts is under the control of the user. Examples are broadband retrieval services for film, high-resolution images, audio information and archival information Distribution services This is modelled on traditional broadcast services and video on demand to distribute information to a large number of users. The requirement of bandwidth and QoS are quite different from interactive services. The distribution services are further divided into the following subclasses: Distribution services without user individual presentation control: these services include broadcast services. They provide a continuous flow of information, which is distributed from a central source to an unlimited number of authorised receivers connected to the network. The user can access this flow of information without the ability to determine at which instant the distribution of a string of information will be started. The user cannot control the start and order of the presentation of the broadcasted information. Depending on the point of time of the user s access, the information will not be presented from the beginning. Examples are broadcast services for television and radio programmes. Distribution services with user individual presentation control: services of this class also distribute information from a central source to a large number of users. However, the information is provided as a sequence of information entities (e.g. frames) with cyclical repetition. So, the user has the ability of individual access to the cyclical distributed information and can control the start and order of presentation. Due to

8 8 Satellite Networking: Principles and Protocols the cyclical repetition, the information entities selected by the user will always be presented from the beginning. One example of such a service is video on demand. 1.4 Internet services and applications Like computers, in recent years the Internet has been developed significantly and the use of it has been extended from research institutes, universities and large organisations into ordinary family homes and small businesses. The Internet was originally designed to interconnect different types of networks including LANs, MANs and WANs. These networks connect different types of computers together to share resources such as memory, processor power, graphic devices and printers. They can also be used to exchange data and for users to access data in any of the computers across the Internet. Today the Internet is not only capable of supporting data, but also image, voice and video on which different network services and applications can be built such as IP telephony, videoconferencing, tele-education and telemedicine. The requirements of new services and applications clearly changed the original objectives of the Internet. Therefore the Internet is evolving towards a new generation to support not only the traditional computer network services but also real-time user services including telephony. Eventually, this will lead to a convergence of the Internet and telecommunication networks towards the future global network infrastructures of which satellite will play an important part World wide web (WWW) The WWW enables a wide range of Internet services and applications including e-commerce, e-business and e-government. It also enables virtual meetings with a new style of work, communication, leisure and lives. The WWW is an application built on top of the Internet, but is not the Internet itself. It can be seen that the basic principle of the Internet hasn t change much in the last 40 years, but applications of the Internet have changed significantly, particularly the user terminals, user software, services and applications, and human computer interface (HCI). The WWW is a distributed, hypermedia-based Internet information system including browsers for users to request information, servers to provide information and the Internet to transport users requests from users to servers and information from servers to users. The hypertext transfer protocol (HTTP) was created in 1990, at CERN, the European particle physics laboratory in Geneva, Switzerland, as a means for sharing scientific data internationally, instantly and inexpensively. With hypertext a word or phrase can contain a link to other text. To achieve this, the hypertext mark up language (HTML), a subset of general mark up language (GML), is used to enable a link within a web page to point to other pages or files in any server connected to the network. This non-linear, non-hierarchical method of accessing information was a breakthrough in information sharing. It quickly became the major source of traffic on the Internet. There are a wide variety of types of information (text, graphics, sounds, movies, etc.). It is possible to use the web to access

9 Introduction 9 information from almost every server connected to the Internet in world. The basic elements for access to the WWW are: HTTP: the protocol used for the WWW to transport web pages. URL (uniform resource locator): defines a format to address the unique location of the web page identified by the IP address of a computer, port number within the computer system and location of the page in the file system. HTML: the programming tags added to text documents that turn them into hypertext documents. In the original WWW, the URL identified a static file. Now it can be a dynamic web page created according to information provided by users; and it can also be an active web page, which is a piece of program code to be downloaded and run on the user s browser computer when clicked File transfer protocol (FTP) FTP is an application layer protocol providing a service for transferring files between a local computer and a remote computer. FTP is a specific method used to connect to another Internet site to receive and send files. FTP was developed in the early days of the Internet to copy files from computer to computer using a command line. With the advent of WWW browser software, we no longer need to know FTP commands to copy to and from other computers, as web browsers have integrated the commands into their browser functions Telnet This is one of the earliest Internet services providing text-based access to a remote computer. We can use telnet in a local computer to login to a remote computer over the Internet. Normally, an account is needed in the remote host so that the user can enter the system. After a connection is set up between the local computer and remote computer, it allows users to access the remote computer as if it were a local computer. Such a feature is called location transparency, i.e., the user cannot tell the difference between the responses from the local machine or remote machine. It is called time transparency if the response is so fast that user cannot tell the difference between local machine and remote machine by response time. Transparency is an important feature in distributed information systems Electronic mail ( ) The is like our postal system but much quicker and cheaper, transmitting only information without papers or other materials, i.e. you can order a pizza through the Internet but cannot receive any delivery from it. The early allowed only text messages to be sent from one user to another via the Internet. can also be sent automatically to a number of addresses. Electronic mail has grown over the past 20 years, from a technical tool used by research scientists, to a business tool as common as faxes and letters. Everyday, millions and millions of s are sent through intranet systems and the Internet. We can also use

10 10 Satellite Networking: Principles and Protocols mailing lists to send an to groups of people. When an is sent to a mailing list, the system distributes the to the listed group of users. It is also possible to send very large files, audio and video clips. The success of systems also causes problems for the Internet, e.g. viruses and junk mail are spread through , threatening the Internet and the many computers linked to it Multicast and content distribution Multicast is a generalised case of broadcast and unicast. It allows distribution of information to multiple receivers via the Internet or intranets. Example applications are content distributions including news services, information on stocks, sports, business, entertainment, technology, weather and more. It also allows real-time video and voice broadcast over Internet. This is an extension to the original design of the Internet Voice over internet protocol (VoIP) VoIP is one of the important services under significant development. This type of service is real time and is more suitable for traditional telecommunication networks. It is different in many ways from the original Internet service. It has quite different traffic characteristics, QoS requirements and bandwidth and network resources. Digitised streams of voices are segmented into voice frames. These frames are encapsulated into a voice packet using a real-time transport protocol (RTP) that allows additional information for real-time service including time stamps to be included. The real-time transport control protocol (RTCP) is designed to carry control and signalling information used for VoIP services. The RTP packets are put into the user datagram protocol (UDP), which is carried through the Internet by IP packets. The QoS of VoIP depends on network conditions in terms of congestion, transmission errors, jitter and delay. It also depends on the quality and available bandwidth of the network such as the bit error rate and transmission speed. Though the RTP and RTCP were originally designed to support telephony and voice services, they are not limited to these, as they can also support real-time multimedia services including video services. By making use of the time-stamp information generated at source by the sender, the receiver is able to synchronise different media streams to reproduce the real-time information Domain name system (DNS) The DNS is an example of application layer services. It is not normally used by users, but is a service used by the other Internet applications. It is an Internet service that translates domain names into IP addresses. Because domain names are alphabetical, they are easier to remember. The Internet, however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name will translate to IP address: The IP address can also be used directly.

11 Introduction 11 The DNS is, in fact, a distributed system in the Internet. If one DNS server does not know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned. The DNS is organised as a hierarchical distributed database that contains mapping of domain names to various types of information including IP addresses. Therefore, the DNS can also be used to discover other information stored in the database. 1.5 Circuit-switching network The concept of circuit-switching networks comes from the early analogue telephony networks. The network can be of different topologies including star, hierarchical and mesh at different levels to achieve coverage and scalability. Figure 1.3 shows typical topologies of networks. An example of telephone networks is shown in Figure 1.4. At local exchange (LEX) level, many telephones connect to the exchange forming a star topology (a complete mesh topology is not scalable). Each trunk exchange (TEX) connects several local exchanges to Figure 1.3 Typical topologies of networks: star, hierarchy and mesh Circuit switching network Top level Trunk Exchanges Local Exchange First level Trunk Exchanges Local Exchange Local Exchange Local Exchange Figure 1.4 Circuit switching networks

12 12 Satellite Networking: Principles and Protocols form the first level of the hierarchy. Depending on the scale of the network, there may be several levels in the hierarchy. At the top level, the number of exchanges is small, therefore a mesh topology is used by adding redundancy to make efficient use of network circuits. All the telephones have a dedicated link to the local exchange. A circuit is set up when requested by a user dialling the telephone number, which signals the network for a connection Connection set up To set up a connection, a set of circuits has to be connected, joining two telephone sets together. If two telephones are connected to the same LEX, the LEX can set up a circuit directly. Otherwise, additional steps are taken at a higher level TEX to set up a circuit across the switching network to connect to the remote LEX then to the destination telephone. Each TEX follows routing and signalling procedures. Each telephone is given a unique number or address to identify which LEX it is connected to. The network knows which TEX the LEX is connected to. The off-hook signal and dialled telephone number provide signalling information for the network to find an optimum route to set up a group of circuits to connect the two telephones identified by the calling telephone number and called telephone number. If the connection is successful, communication can take place, and the connection is closed down after communication has ended. If the connection fails or is blocked due to lack of circuits in the network, we have to try again. At this point, you may imagine that due to the wide coverage of satellite systems, it is possible to have satellites acting as a LEX to connect the telephones directly, or to act as a link to connect LEX to TEX, or connect TEX together. The roles of the satellite in the network have a significant impact on the complexity and cost of the satellite systems, as the different links require different transmission capacities. Satellites can be used for direct connection without strict hierarchy for the scalability needed in terrestrial networks Signalling Early generation of switches could only deal with very simple signalling. Signalling information was kept to the minimum and the signal used the same channel as the voice channel. Modern switches are capable of dealing with a large amount of channels, hence the signalling. The switches themselves have the same processing power as computers, are very flexible and are capable of dealing with data signals. This leads to separation of signal and user traffic, and to the development of common channel signalling (CCS). In CCS schemes, signals are carried by the same channel over a data network, separated from the voice traffic. Combination of the flexible computerised switch and CCS enables a better control and management of the telephone network and facilitates new services such as call forwarding, call back and call waiting. Signalling between network devices can be very fast, but responses from people are still the same. The processing power of devices can be improved significantly but not people s ability to react. People used to cause stress to network technologies, but now they are often stressed by technologies.

13 Introduction Transmission multiplexing hierarchy based on FDM Frequency division multiplexing (FDM) is a technique to share bandwidth between different connections in the frequency domain. All transmission systems are design to transmit signals within a bandwidth limit measured in hertz (Hz). The system may allocate a fraction of the bandwidth-called channel to a connection to support a network service such as telephony rather than allocate a physical cable to the connection. This effectively increases the capacity. When the bandwidth is divided into channels, each channel can support a connection. Therefore, connections from many physical links can be multiplexed into a single physical link with many channels. Similarly, multiplexed connections in one physical connection can be de-multiplexed into many physical connections. Figure 1.5 illustrates the concept of multiplexing in the frequency domain. The given channel can be used to transmit digital as well as analogue signals. However, analogue transmission is more convenient to process in the frequency domain. A traditional telephone channel transmits audio frequency at a bandwidth of 3.1 khz (from 0.3 to 3.4 khz). It is transmitted in the form of a single-sideband (SSB) signal with suppressed carriers at 4 khz spacing. Through multiplexing, 12 or 16 single channels can form a group. Five groups can form a super-group, super-group to master-group or hyper-group, and to super-group and master-group. Figure 1.6 shows the analogue transmission hierarchy Transmission multiplexing hierarchy based on TDM Digital signals can be processed conveniently in the time domain. Time division multiplexing (TDM) is a technique to share bandwidth resources in the time domain. A period of time called a frame can be divided into time slots. Each time slot can be allocated to a connection. The frame can support the same number of connections as the number of slots. For example, the basic digital connection for telephony is 64 kbit/s. Each byte will take 125 microseconds to transmit. If the transmission speed is very fast, each byte can be transmitted in a fraction frequency time frequency time Multiplexor time time Figure 1.5 Concept of multiplexing in the frequency domain

14 14 Satellite Networking: Principles and Protocols 4 khz per channel ( khz) Channel 1 Channel 2 Channel 3 48 khz per groups ( khz) Group 1 (12 Channels) Channel 4 Channel 5 Group 2 (12 Channels) 16 X Super-group (9600 Channels) Channel 6 Channel 7 Group 3 (12 Channels) Super-group (60 Channels) Master-group (300 Channels) Channel 8 Channel 9 Channel 10 Channel 11 Group 4 (12 Channels) Group 5 (12 Channels) 12 MHz (2700 Channels) Hyper-group (900 Channels) 60 MHz (10800 Channels) Channel 12 Figure 1.6 Analogue transmission multiplexing hierarchy of the 125 microseconds, and then a time frame of 125 microseconds can be divided into more time slots to support one connection for each slot. Several slow bit streams can be multiplexed into one high-speed bit stream. Figure 1.7 illustrates the concept of multiplexing in the time domain. frequency time Multiplexor frequency time Figure 1.7 Concept of multiplexing in the time domain

15 Introduction 15 North America X X X X kbit/s X3 X3 Europe X X X X X Figure 1.8 Digital transmission hierarchies The digital streams in the trunk and access links are organised into the standard digital signal (DS) hierarchy in North America: DS1, DS2, DS3, DS4 and higher levels starting from Mbit/s; in Europe, they are organised into E1, E2, E3, E4 and higher levels starting from Mbit/s. The two hierarchies can only internetwork at certain levels, however, the basic rate is the same 64 kbit/s needed to accommodate one telephone circuit. Additional bits or bytes are added to the multiplexed bit stream for signalling and synchronisation purposes, which are also different between North America and European systems. Figure 1.8 shows the transmission multiplexing hierarchies Space switching and time switching In telephony networks and broadcasting networks, the usage of each channel normally is in the order of minutes or hours. The requirements for bandwidth resources are also well defined. For example, channels for telephony services and broadcast services are all well defined. If a switch cannot buffer any information, space in terms of bandwidth or time slots has to be reserved to allow information to flow and switched across the switch as shown in Figure 1.9. This means that the switch can only perform space switching. If a switch can buffer a frame of time slots, the output of slot contents in the frame can be switched as shown in Figure This means that the switch can perform time switching Switching controller Switching fabrics Switching table: 1 to 4 2 to 1 3 to 2 4 to 3 Figure 1.9 Space switching concept

16 16 Satellite Networking: Principles and Protocols Switching logics Tailer Time slots: Header Time slots: d c b a Buffers a d c b Time frame Time frame after switching Figure 1.10 Time switching concept Switch designs can use either/or a combination of space switching and time switching, such as space-time-space or time-space-time combinations Coding gain of forward error correction (FEC) In satellite networking, the transmission from satellite to the earth station is normally power limited. To make it worse, there may be propagation loss and increased noise power. Therefore, it is important to introduce an error correction coding, i.e., to add additional information to the data so that some errors can be corrected by the receiver. This is called forward error correction (FEC), because the additional information and processing take place before any error occurs. Depending on modulation schemes, bit error probability (BEP) is expressed as a function of E b /N 0 which is related to E c /N 0 by expression: E b /N 0 = E c /N 0 10 log (1.1) where E b is the energy per bit without coding, E c is the energy per bit with coding, N 0 is the noise spectral density (W/Hz) and = n/ n + r is the code rate (where r is the number of bits added for n information bits). It can be seen that we can use less power to improve the BEP at the cost of additional bits (hence bandwidth). The value (10 log ) is called the coding gain. There is also a trade-off between power and bandwidth for a given BEP. Using C = E c R c, we calculate: where C is carrier power, and R c is the channel bit rate. E c /N 0 = C/R c /N 0 = C/N 0 /R c (1.2) 1.6 Packet-switching networks The packet switching concept was developed for computer networks, because streams of bits or bytes do not make much sense to computers. The computer needs to know the start and end of the data transmission. In a data network, it is important to be able to identify where transmission of data starts and where transmission ends. The data, together with identifiers of the start and end of the data, is called a frame. In addition, addresses, frame checks and other information are added so that the sending computer can tell the receiving computer what to do based on a protocol

17 Introduction 17 sent when the frame is received. If the frame is exchanged on a link between two computers, it is defined by the link layer protocol. The frame is special packet on links. Therefore, the frame is related to link layer functions. Information can also be added to the frame to create a packet so that the computer can make use of it to route the packet from the source to the destination across the network. Therefore, the packet is related to network layer functions. The initial packet network was design for transmission of messages or data. The start and end of the data, correctness of transmission and mechanisms to detect and recover errors are all important. If the communication channel is perfect, a complete message can be handled efficiently as a whole, however, in the real world, this assumption cannot be met easily. Therefore, it is practical to break down the message into smaller segments using packets for transmission. If there is any error in the message, only the error packet needs to be dealt with rather than the whole message. With packets, we don t need to divide bandwidth resources into narrow channels or small time slots to meet service requirement. We can use the complete bandwidth resources to transmit packets at high speed. If we need more bandwidth, we can simply use more or larger packets to send our data. If we use less bandwidth, we use fewer and smaller packets. Packets provide flexibility for bandwidth resource allocations, particularly when we don t know the requirement of bandwidth resources from some new multimedia services. The meaning of broadband has been defined by the ITU-T as a system or transmission capable of dealing with data rates higher than the primary rates, which are Mbit/s in North America and Mbit/s in Europe. There are two approaches for the packet-switching network. One is used in traditional telephony networks and the other is used in the computer and data networks Connection-oriented approach In a packet-switching network, each physical connection has a much wider bandwidth, which is capable of supporting high-speed data transmissions. To divide this bandwidth for more connections, the concept of a virtual channel is used. The packet header carries an identification number to identify different logical connections within the same physical connection. On receiving the packet, the packet switch can forward the packet to the next switch using another virtual channel until the packet reaches its destination. For switching, the network needs to be set up before the packet is transmitted. That is, a switching table needs to be set up in the switch to connect the incoming virtual channels to the outgoing virtual channels. If connection requirements are known, the network can reserve resources for the virtual connections in terms of packets and their payload. This approach is called the virtual channel approach. Like telephony networks, the virtual channel based approach is connection oriented, i.e., a connection needs to be set up before communication. All packets follow the same connection from source to destination. The connection is called virtual connection. In circuit switching, physical paths are set up to switch from input channels to output channels. In virtual channel switching, channels are identified by logic numbers; hence changing the logic number identifier virtually switches the packets to a different logical channel. Virtual channel switching is also called virtual circuit switching. Figure 1.11 illustrates the concept of virtual channel switching.

18 18 Satellite Networking: Principles and Protocols Vitual channel identifier Payload Header New virtual channel ID in1 out1 Buffers & Processor in2 out2 Packets Packets with new IDs Switching table: in1:1 -> out1:5 in1:2 -> out2:1 in1:3 -> out2:2 in1:4 -> out2:3 in2:1 -> out1:4 in2:2 -> out1:1 in2:3 -> out1:2 in2:4 -> out2:6 Figure 1.11 Virtual channel switching concept The network node is called a packet switch, and functions like traditional circuit switching, but it gives flexibility of allocating different amounts of resources to each virtual connection. Therefore it is a useful concept for a broadband network, and is used in the asynchronous transfer mode (ATM) network. The virtual connection identifiers are only significant to each switch for identifying logical channels. This kind of network is quite similar to our telephony and railway networks. Resources can be reserved to guarantee QoS during the connection set-up stage. The network blocks the connection request if there are not enough resources to accommodate the additional connection Connectionless approach In computer and data networks, transmission of information often takes a very short period of time compared to telephone connections. It becomes inefficient to set up a connection for the computer and data networks for each packet transmission. To overcome the problem with the virtual channel approach, the connectionless approach is used to transmit packets from sources to destinations without pre-setting connections. Such a packet is called the datagram approach because it consists of source and destination addresses rather than connection identifiers to allow the network node (also called the router) to route the packet from source to destination. Figure 1.12 illustrates the concept of connectionless approach. In a connectionless network, the packet header needs to carry the destination address so that the network can use it to route the packet from source to destination, and also the source address for response by the destination computer. The network packet switch is called a router to distinguish it from the connection-oriented switch or traditional channel-based

19 Introduction 19 Destination address Payload Header c z b y Datagram packets a x Buffers & Processor net1 net2 z c b y x a Routing table: a -> net2 b -> net1 c -> net2 u -> net1 v -> net2 w -> net1 x -> net1 y -> net1 z -> out2 Figure 1.12 Datagram routing concept switch. The router has a routing table containing information about destination and the next node leading to the destination with minimum costs. The connectionless approach has flexibility for individual packets to change to different routes if there is congestion or failure in the route to destination. This kind of network is quite similar to postal delivery and motorway networks in the UK. There is no way to make a reservation, hence there is no guarantee of QoS. When traffic conditions are good, one car journey can give a good estimate of travel time. Otherwise, it may take much more time to reach the destination and sometimes it can be too late to be useful. However, there is flexibility to change its route after starting the journey to avoid any congestion or closure in the route. The Internet is an example of this kind of network, hence the information highway is a good description of the information infrastructure widely used today Relationship between circuit switching and packet switching Circuit switching relates more closely to transmission technologies than packet switching. It provides physical transmission of signals carrying information in the networks. The signals can be analogue and digital. For analogue signals it provides bandwidth resources in term of Hz, khz or MHz, treated in the frequency domain such as FDM; and for digital signals it provides bandwidth resources in term of bit/s, kbit/s or Mbit/s, treated in the time domain such as TDM. It is also possible to take into account both time and frequency domains such as CDMA. At this level, switches deal with streams of bits and bytes of digital signals to flow along the circuits or analogue signals with defined bandwidth. There is no structure in the signal. Packets provide a level of abstraction above the bit or byte level, by providing structure to bit streams. Each packet consists of a header and payload. The header carries information

20 20 Satellite Networking: Principles and Protocols to be used by the network for processing, signalling, switching and controlling purposes. The payload carries information to be received and processed by user terminals. On top of a circuit it is possible to transmit packets. With packets it is possible to emulate the circuit by continuous streams of packets. These allow internetworking between circuit networks and packet networks. The emulated circuit is called a virtual circuit. It can be seen that virtual circuit, frame and packet are different levels of abstract from physical transmissions to network layer functions Impacts of packet on network designs A packet is a layer of functions introduced to the networks. It separates the user services and applications from transmission technologies. A packet provides flexibility for carrying voice, video and data without involving transmission technologies and media. The network only deals with packets rather than different services and applications. The packets can be carried by any network technology including satellite. Introducing packets into networks brings tremendous benefit for developing new services and applications and for exploring new network technologies, and also brings a great challenge to network designers. What size should the packet be? There should be a trade-off between requirements from applications and services and the capabilities of transmission technologies. If is too small, it may not be capable of meeting the requirements, but if it is too big it may not be fully utilised and may also cause problems in transmission. Large packets are more likely to get bit errors than small ones, as transmission channels are never perfect in real life. For large packets it takes a long time to transmit and process and they also need large memory space to buffer them. Real-time services may not be able to tolerant long delays, hence there is a preference for small packets Packet header and payload How many bits should be used for the packet header and how many for payload? With a large header, it is possible to carry more control and signal information. It also allows more bits to be used for addresses for end systems, but it can be very inefficient if services need only a very small payload. There are also special cases for large headers, for example, a large header may be needed for secure transmission of credit card transactions Complexity and heterogeneous networks The complexity is due to a large range of services and applications and different transmission technologies. Many different networks have been developed to support a wide range of services and applications and to better utilise bandwidth resources based on packet-switching technologies. Systems may not work together if they are developed with different specifications of packets. Therefore such issues have to be dealt with in a much wider community in order for systems to interwork globally. This is often achieved by developing common international standards.

21 Introduction Performance of packet transmissions At bit or byte level, transmission errors are overcome by increasing transmission power and/or bandwidth using better channel coding and modulation techniques. In real systems, it is impossible to eliminate bit errors completely. The errors at bit level will propagate to packet levels. Retransmission mechanisms are used to recover the error/lost packets, thus controlling the error at packet levels. Therefore, packet transmission can be made reliable even if bit transmissions are unreliable. However, this additional error recovery capability is at the cost of additional transmission time and buffer space. It also relies on efficient error detection schemes and acknowledgement packets to confirm a successful transmission. For the retransmission scheme, the efficiency of channel utilisation can be calculated as: = t t / t t + 2t p + t r (1.3) where t t is the time for transmission of a packet onto the channels, t p is the time for propagation of the packet along the channel to the receiver, and t r is the processing time of the acknowledgement packet by the receiver. It can be seen that large packet transmission times or small propagation times and packet processing times are good for packet transmission performance Impact of bit level errors on packet level We may quickly realise that a large packet can also lead to a high probability of packet error. If P b is the probability of a bit error, the probability of packet error P p of n bits can be calculated as: P p = 1 1 P b n (1.4) Figure 1.13 shows the packet error probabilities for given bit error probabilities and packet sizes. Packet error probability 1.0E E E E E E - 09 Bit error probabilities 1.00E E E E E E - 11 Figure Packet size (bit) Packet error probabilities for given bit error probabilities and packet sizes

22 22 Satellite Networking: Principles and Protocols 1.7 OSI/ISO reference model Protocols are important for communications between entities. There are many options available to set protocols. For global communications, protocols are important to be internationally acceptable. Obviously, the International Standards Organisation (ISO) has played a very important role in setting and standardising a reference model so that any implementations following the reference model will be able to internetwork and communicate with each other. Like any international protocol, it is easy to agree in principle how to define the reference model but always difficult to agree about details such as how many layers the model should have, how many bytes a packet should have, how many headers a packet should have to accommodate more functionalities but minimise overheads, whether to provide best-effort or guaranteed services, whether to provide connection-oriented services or connectionless services, etc. There are endless possible options and trade-offs with many technological selections and political considerations Protocol terminology A protocol is the rules and conventions used in conversation by agreement between the communicating parties. A reference model provides all the roles so that all parties will be able to communicate with each other if they follow the roles defined in the reference model in their implementation. To reduce design complexity, the whole functions of systems and protocols are divided into layers, and each layer is designed to offer certain services to higher layers, shielding those layers from the details of how the services are actually implemented. Each layer has an interface with the primitive operations, which can be used to access the offered services. Network protocol architecture is a set of layers and protocols. A protocol stack is a list of protocols (one protocol per layer). An entity is the active element in each layer, such as user terminals, switches and routers. Peer entities are the entities in the same layer capable of communication with the same protocols. Basic protocol functions include segmentation and reassembly, encapsulation, connection control, ordered delivery, flow control, error control, and routing and multiplexing. Protocols are needed to enable communicating parties to understand each other and make sense of received information. International standards are important to achieve a global acceptance. Protocols described in the standards are often in the context of reference models, as many different standards have been developed Layering principle The layering principle is an important concept for network protocols and reference models. In the 1980s, the ISO derived the seven-layer reference model shown in Figure 1.14 called the open systems interconnection (OSI) reference model, which is based on clear and simple principles. It is the first complete reference model developed as an international standard. The principles that were applied to arrive at the seven layers can be summarised as: A layer defines a level of abstraction which should be a different from any other layer. Each layer performs a well-defined function.

23 Introduction 23 User terminal 7. Application 6. Presentation 5. Session 4. Transport Application protocol Presentation protocol Session protocol Transport protocol Network boundary Internal network protocols User terminal 7. Application 6. Presentation 5. Session 4. Transport 3. Network 3. Network 3. Network 3. Network 2. Data Link 2. Data Link 2. Data Link 2. Data Link 1. Physical 1. Physical 1. Physical 1. Physical User terminal to network protocols at different layers Figure 1.14 OSI/ISO seven-layer reference model The function of each layer should be chosen to lead to internationally standardised protocols. The layer boundaries should be chosen to minimise information flow across the interface. The number of layers should be large enough but not too large Functions of the seven layers The following are brief descriptions of the functions of each layer. Layer 1 the physical layer (bit stream) specifies mechanical, electrical and procedure interfaces and the physical transmission medium. In satellite networks, radio links are the physical transmission media; modulation and channel coding enable the bit stream to be transmitted in defined signals and allocated frequency bands. Layer 2 the data link layer provides a line that appears free of undetected transmission errors to the network layer. Broadcasting media have additional issues in data link layer, i.e., how to control access to the shared medium. A special sublayer called the medium access control (MAC) schemes, such as Polling, Aloha, FDMA, TDMA, CDMA, DAMA, deals with this problem. Layer 3 the network layer routes packets from source to destination. The functions include network addressing, congestion control, accounting, disassembling and reassembling, coping with heterogeneous network protocols and technologies. In broadcast networks, the routing problem is simple: the routing protocol is often thin or even non-existent.

24 24 Satellite Networking: Principles and Protocols Layer 4 the transport layer provides a reliable data delivery service for high layer users. It is the highest layer of the services associated with the provider of communication services. The higher layers are user data services. It has functions of ordered delivery, error control, flow control and congestion control. Layer 5 the session layer provides the means of cooperating presentation entities to organise and synchronise their dialogue and to manage the data exchange. Layer 6 the presentation layers are concerned with data transformation, data formatting and data syntax. Layer 7 the application layer is the highest layer of the ISO architecture. It provides services to application processes Fading of the OSI/ISO reference model Today we can see the development of many types of new applications, services, networks and transmission media. No one expected such a fast development of the Internet and new services and applications. New technologies and new service and application developments have changed the conditions of the optimisation points of the layering functions as one of the reasons leading to the fading of the international standards. There are also many other reasons, including technical, political and economical reasons, or too complicated to be used in a practical world. The reference model is not much used in today s networks. However, the principles of layering protocol are still widely used in network protocol design and implementation. It is the classical and true reference model that all modern protocols always try to use as a reference to discuss and describe the functions of their protocols and evaluate their performance by analysis, simulation and experiment. 1.8 The ATM protocol reference model The asynchronous transfer model (ATM) is based on fast packet switching techniques for the integration of telecommunications and computer networks. Historically, telephone networks and data networks were developed independently. Development of integrated services digital network (ISDN) standards by the ITU-T was the first attempt to integrate telephony and data networks Narrowband ISDN (N-ISDN) N-ISDN provides two 64 kbit/s digital channels, which replace the analogue telephone services plus a 16 kbit/s data channel for signalling and data services from homes to local exchanges. The ISDN follows the concept of circuit networks very closely, as the envisaged main services, telephony and high-speed data transfer, need no more than 64 kbit/s. The primary rates are 1.5 Mbit/s for North America and 2 Mbit/s for Europe Broadband ISDN (B-ISDN) ATM is a further effort by ITU-T to develop a broadband integrated services digital network (B-ISDN) following the development of ISDN, which is called narrowband ISDN (N-ISDN) to distinguish it from B-ISDN.

25 Introduction 25 As soon as standardisation of the N-ISDN was complete, it was realised that the N- ISDN based on circuit networks could not meet the increasing demand by new services and applications and data networks. The standardisation processes of B-ISDN led to the development of ATM based on the concept of packet switching. It provides flexibility of allocating bandwidth to user services and applications from tens of kbit/s used for telephony services to hundreds of Mbit/s for high-speed data and high definition TV. The ITU-T recommended that the ATM is the target solution for broadband ISDN. It is the first time in its history that standards were set up before development ATM technology The basic ATM technology is very simple. It is based on a fixed packet size of 53 bytes of which 5 bytes are for the header and 48 for payload. The ATM packet is called a cell, due to the small and fixed size. It is based on the virtual channel switching approach providing a connection-oriented service and allowing negotiation of bandwidth resources and QoS for different applications. It also provides control and management functions to manage the systems, traffic and services for generating revenue from the network operations The reference model The reference model covers three plans: user, control and management. All transportation aspects are in the form of ATM, as shown in Figure 1.15 including the: physical layer provides physical media-related transmissions such as optical, electrical and microwave; ATM layer defines ATM cells and related ATM functions; and ATM adaptation layer adapts high-layer protocols including the services and applications and divides data into small segments so that they can be suitable for transportation in the ATM cells. Management Plane Control Plane User Plane Higher layers Higher layers ATM Adaptation Layer Layer Management Plane Management ATM Layer Physical Layer Figure 1.15 B-ISDN ATM reference model

26 26 Satellite Networking: Principles and Protocols Problems: lack of available services and applications The ATM has been influenced by the development of optical fibre, which provides very large bandwidths and very low transmission errors. However, such transmission conditions are hardly possible in satellite transmission systems. Services and applications are considered as parts of functions in user terminals rather than as parts of the network. The networks are designed to be able to meet all the requirements of services and applications. However, the higher layers were never defined and so few services and applications were developed on the ATM network. ATM has tried to internetwork with all different sorts of networks including some legacy networks together with the management and control functions making ATM very complicated and expensive to implement. 1.9 Internet protocols reference model Originally, the Internet protocols were not developed by any international standardisation organisation. They were developed by the Department of Defense (DoD) research project to connect a number of different networks designed by different vendors into a network of networks (the Internet ). It was initially successful because it delivered a few basic services that everyone needed (file transfer, electronic mail, telnet for remote logon) across a very large number of different systems. The main part of the Internet protocol reference model is the suite of transmission control protocol (TCP) and Internet protocol (IP) known as the TCP/IP protocols. Several computers in a small department can use TCP/IP (along with other protocols) on a single LAN or a few interconnected LANs. The Internet protocols allow the construction of very large networks with less central management. As all other communications protocol, TCP/IP is composed of different layers but is much simpler than the ATM. Figure 1.16 shows the Internet reference model. Layers (OSI) Application HTTP SMTP FTP Telnet DNS RTP/RTCP etc... Transport TCP / UDP Network IP Link + Physical Satellite Network Wireless LAN Ethernet LAN ATM Network etc.... Figure 1.16 The Internet reference model

27 Introduction Network layer: IP protocol The network layer is the Internet protocol (IP) based on the datagram approach, proving only best effort service without any guarantee of quality of service. IP is responsible for moving packets of data from node to node. IP forwards each packet based on a four-byte destination address (the IP address). The Internet authorities assign ranges of numbers to different organisations. The organisations assign groups of their numbers to departments Network technologies The network technologies, including satellite networks, LANs, ATM, etc., are not part of the protocols. They transport IP packets from one edge of the network to the other edge. The source host sends IP packets and the destination host receives the packets. The network nodes route the IP packets to the next routers or gateways until they can route the packets directly to the destination hosts Transport layer: TCP and UDP The transmission control protocol (TCP) and user datagram protocol (UDP) are transport layer protocols of the Internet protocol reference model. They provide ports or sockets for services and applications at user terminals to send and receive data across the Internet. The TCP is responsible for verifying the correct delivery of data between client and server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received. Therefore TCP provides a reliable service though the network underneath may be unreliable, i.e., operation of Internet protocols do not require reliable transmission of packets, but reliable transmission can reduce the number of retransmissions and hence increase performance. UDP provides the best-effort service without trying to recover any error or loss. Therefore, it is also a protocol providing unreliable transport of user data. However, this is very useful for real-time application, as retransmission of any packet may cause more problems than the lost packets Application layer The application layer protocols are designed as functions of the user terminals or server. The classical Internet application layer protocols include HTTP for WWW, FTP for file transfer, SMTP for , telnet for remote login, DNS for domain name service and more including real-time protocol (RTP) and real-time control protocol (RTCP) for real-time services and others for dynamic and active web services. All these should be independent from the networks Problems: no QoS and no control on resources Most functions of the Internet define the high layer protocols. Current Internet protocol version 4 (IPv4) provides only best-effort services, hence it does not support any control functions and cannot provide any quality of services. The problems are addressed in the next generation of the Internet protocol version 6 (IPv6).

28 28 Satellite Networking: Principles and Protocols 1.10 Satellite network There are two types of transmission technologies: broadcast and point-to-point transmissions. Satellite networks can support both broadcast and point-to-point connections. Satellite networks are most useful where the properties of broadcast and wide coverage are important. Satellite networking plays an important role in providing global coverage. There are three types of roles that satellites can play in communication networks: access network, transit network and broadcast network Access network The access network provides access for user terminals or private networks. Historically in telephony networks, it provided connections from telephone or private branch exchanges (PBX) to the telephony networks. The user terminals link to the satellite earth terminals to access satellite links directly. Today, in addition to the telephony access network, the access networks can also be the ISDN access, B-ISDN access and Internet access Transit network The transit network provides connection between networks or network switches. It often has a large capacity to support a large number of connections for network traffic. Users do not have direct access to it. Therefore they are often transparent to users, though they may notice some differences due to propagation delay or quality of the link via a satellite network. Examples of satellite as transit networks include interconnect international telephony networks, ISDN, B-SDN and Internet backbone networks. Bandwidth sharing is often pre-planned using fixed assignment multiple access (FAMA) Broadcast network Satellite supports both telecommunication service and broadcast service. Satellite can provide very efficient broadcasting services including digital audio and video broadcast (DVB-S) and DVB with return channels via satellite (DVB-RCS) Space segment The main components of a communication satellite system consist of the space segment: satellites, and the ground segment: earth stations. The design of satellite networks is concerned with service requirements, orbit and coverage and frequency band selection (see Figure 1.17). The satellite is the core of the satellite network consisting of a communication subsystem and platform. The platform, also called a bus, provides the structure support and power supply of the communication subsystems, and also includes altitude control, orbit control, thermal control, tracking, telemetry and telecommand (TT&T) to maintain normal operations of the satellite system.

29 Introduction 29 Propulsion system Solar arrays Telemetry, altitude control, commanding, fuel, battery & power Solar arrays Space Segment Down converter, preamplifier and filter Optional: on board processing, switching or on routing High power amplifier (HPA) and filter Space Segment Ground Segment Ground Segment Network Terminals Satellite Control Centre (SCC) Network Control Centre (NCC) Network Figure 1.17 Illustration of the space segment and ground segment The telecommunication subsystems consist of transponders and antenna. The antennas associated with the transponders are specially designed to provide coverage for the satellite network. Modern satellites may also have onboard processing (OBP) and onboard switching (OBS). There are different types of transponders: Transparent transponders provide the function of relaying radio signals. They receive transmissions from the earth station and retransmit them to the earth station after amplification and frequency translation. Satellites with transparent transponders are called transparent satellites. OBP transponders provide addition functions including digital signal processing (DSP), regeneration and base band signal processing before retransmitting the signal from satellite to the earth station. Satellites with OBP transponders are called OBP satellites. OBS transponders have additional functions than OBP transponders, providing switching functions. Similarly, satellites with OBS transponders are called OBS satellites. With the rapid development of the Internet, experiments are also in progress to fly onboard routers. In addition, the satellite control centre (SCC) and network control centre (NCC) or network management centre (NMC), are parts of the space segment, though they are located at ground level: Satellite control centre (SCC): it is the on-ground system responsible for the operation of the satellite. It monitors the status of the different satellite subsystems through telemetry links, controls the satellite on its nominal orbit through telecommand links. It communicates with the satellite using dedicated links, which are different from the communication

30 30 Satellite Networking: Principles and Protocols links. It normally consists of typically one earth station and GEO or non-geo satellite systems, receiving telemetry from the satellites and sending telecommands to the satellites. Sometimes, a backup centre is built at a different location to improve reliability and availability. Network control centre (NCC) or network management centre (NMC): this has different functions from the SCC. Its main functions are to manage the network traffic and associated resources on board the satellite and on ground to achieve efficient use of the satellite network for communications Ground segment The earth station is part of the satellite network. It provides functions of transmitting and receiving traffic signals to and from satellites. It also provides interfaces to terrestrial networks or to user terminals directly. The earth station may consist of the following parts: The transmitting and receiving antenna are the most visible parts of the earth station. There are different sizes typically ranging from below 0.5 metres to 16 metres and above. Low noise amplifier of the receiver system with noise temperature ranging from about 30 K to a few hundred K. High performance amplifier (HPA) of the transmitter with power from a few watts to a few thousands kilowatts depending on capacity. Modulation, demodulation and frequency translation. Signal processing. Interfaces to terrestrial networks or user terminals Satellite orbits Orbits are one of the importance resources for satellite in space, as satellites need to be in a right orbit to provide coverage to the service areas. There are different ways to classify satellite orbits (see Figure 1.18). According to the altitude of satellites, satellite orbits can be classified as the following types: Low earth orbit (LEO) has an altitude range of less than 5000 km. Satellites in this type of orbit are called LEO satellites. The period of the satellite is about 2 4 hours. Media earth orbit (MEO) has an altitude range between 5000 to km. Satellites in this type of orbit are called MEO satellites. The period of the satellite is about 4 12 hours. Highly elliptical earth orbit (HEO) has an altitude range of more than km. Satellites in this type of orbit are called HEO satellites. The period of the satellite is more than 12 hours. Please note that the space surrounding the earth is not as empty as it looks. There are mainly two kinds of space environment constraints to be considered when choosing orbit altitude.

31 Introduction 31 Highly Elliptical Earth Orbit (HEO) Low Earth Orbit (LEO) Medium Earth Orbit (MEO) Earth Inner Van Allen Radiation belt Outer Van Allen Radiation belt Geostationary Orbit (GEO) Figure 1.18 Satellite orbits The Van Allen radiation belts where energetic particles such as protons and electrons are confined by the earth s magnetic field. They can cause damage to the electronic and electrical components of the satellite. Space debris belts where spacecraft are abandoned at end of their lifetime. They are becoming of increasing concern to the international community as they can also cause damage to satellite networks particularly satellite constellations and to space missions in the future Satellite transmission frequency bands Frequency bandwidth is another important resource of satellite networking and also a scarce resource. The radio frequency spectrum extends from about 3 khz to 300 GHz, communications above 60 GHz are generally not practical because of the high power needed and equipment costs. Parts of this bandwidth are used for terrestrial microwave communication links historically, and for terrestrial mobile communications such as GSM and 3G networks and wireless LANs today. In addition, the propagation environment between the satellite and earth station due to rain, snow, gas and other factors and limited satellite power from solar and battery limits further suitable bandwidth for satellite communications. Figure 1.19 shows attenuations of different frequency bands due to rain, fog and gas. Link capacity is limited by the bandwidth and transmission power used for transmission. Frequency bandwidths are allocated by the ITU. There are several bands allocated for satellite communications. Table 1.1 shows the different available bandwidths for satellite communications. Historically, bandwidths around 6 GHz for uplink and 4 GHz for downlink have been commonly paired in the C band. Many FSS still use these bands. Military and governmental systems use bands around 8/7 GHz in the X band. There are also some systems that operate around 14/12 GHz in the Ku band. New-generation satellites try to use the Ka band to explore wide bandwidth due to saturation of the Ku band. Table 1.2 gives examples of uses of frequency bands.

32 32 Satellite Networking: Principles and Protocols Loss (db) mm/h A A A 25 mm/h 5 mm/h C 0.25 mm/h B A Frequency (GHz) 0.1 g/m Figure 1.19 Attenuations of different frequency bands due to A: rain, B: fog and C: gas Table 1.1 Typical frequency bands of satellite communications Denomination Frequency bands (GHz) UHF L band S band C band X band Ku band K band Ka band Characteristics of satellite networks Most of the presently employed communication satellites are radio frequency (RF) repeaters or bent pipe satellites. A processing satellite, as a minimum, regenerates the received digital signal. It may decode and recode a digital bit stream. It also may have some bulk switching capability and inter satellite links (ISL). Radio link (microwave LOS) provides real transmission of the bits and bytes at the physical layer of the layered reference model. There are three basic technical problems in the satellite radio link due to the satellite being located at great distances from the terminal earth stations.

33 Introduction 33 Table 1.2 Example usages of frequency bands for GEO Denomination Uplink (bandwidth) Downlink (bandwidth) Typical utilisation in FSS for GEO 6/4 C band (575 MHz) (575 MHz) International and domestic satellites: Intelsat, USA, Canada, China, France, Japan, Indonesia 8/7 X band (500 MHz) (500 MHz) Governmental and military satellites International and domestic satellites in Region 1 and (1000 MHz) Intelsat, Eutelsat, France, German, Spain, Russia 13 14/11 12 Ku band (750 MHz) International and domestic satellites in Region (700 MHz) Intelsat, USA, Canada, Spain 18/ (800 MHz) BSS bands Feeder link for BSS 30/20 Ka band (2500 MHz) (2500 MHz) International and domestic satellites Europe, USA, Japan 40/20 Ka band (3000 MHz) 18.2, 21.2 (3000 MHz) Governmental and military satellites Propagation delay The first problem to deal with is very long distances. For GEO satellites, the time required to traverse these distances namely, earth station to satellite to another earth station is in the order of 250 ms. Round-trip delay will be of or 500 ms. These propagation times are much greater than those encountered in conventional terrestrial systems. One of the major problems is propagation time and resulting echo on telephone circuits. It delays the reply of certain data circuits for block or packet transmission systems and requires careful selection of telephone signalling systems, or call set-up time may become excessive Propagation loss and power limited The second problem is that there are far greater losses. For LOS microwave we encounter free-space losses possibly as high as 145 db. In the case of a satellite with a range of miles operating on 4.2 GHz, the free-space loss is 196 db and at 6 GHz, 199 db. At 14 GHz the loss is about 207 db. This presents no insurmountable problem from earth to satellite,

34 34 Satellite Networking: Principles and Protocols where comparatively high-power transmitters and very high-gain antennas may be used. From satellite to earth the link is power-limited for two reasons: 1. In bands shared with terrestrial services, such as the popular 4-GHz band, to ensure non-interference with those services; and 2. In the satellite itself, which can derive power only from solar cells. It takes a great number of solar cells to produce the RF power necessary; thus the downlink, from satellite to earth, is critical, and received signal levels will be much lower than on comparative radio links, as low as 150 dbw Orbit space and bandwidth limited for coverage The third problem is crowding. The equatorial orbit is filling up with geostationary satellites. Radio-frequency interference from one satellite system to another is increasing. This is particularly true for systems employing smaller antennas at earth stations with their inherent wider beam widths. It all boils down to a frequency congestion of emitters Operational complexity for LEO In addition to the GEO satellite, we also see several new low earth orbit satellite systems in operation, which can explore the potential of satellite capabilities. These satellites typically have much lower altitude orbits above the earth. This may reduce the problems of delay and loss, but introduce more complexity in maintaining communication links between earth terminals and satellites due to the fast movement of LEO constellation satellites Channel capacity of digital transmissions In the frequency domain, greater bandwidth can support more communication channels. In the time domain, the digital transmission capacity is also directly proportional to the bandwidth The Nyquist formula for noiseless channels For a noiseless channel, the Nyquist formula is used to determine the channel capacity: C = 2B log 2 M (1.5) where C is the maximum channel capacity for data transfer rate in bit/s, B is bandwidth in hertz and M is the number of levels per signalling element The Shannon theorem for noise channels The Shannon and Hartley capacity theorem is used to determine the maximum bit rate C over a band-limited channel giving a specific signal-to-noise ratio (S/N). The theorem is: C = B log S/N (1.6)

35 Introduction 35 where C is the maximum capacity in bit/s, B is bandwidth of the channel, S is signal power and N is noise power. As S = RE b and N = N 0 B the formula can be rewritten in a different form as the following: C = B log RE b / N 0 B = B log R/B E b /N 0 (1.7) where E b is energy per bit, R is transmission bit rate and N = N 0 B where N 0 is noise power spectral density Channel capacity boundary Let R = C in Equation (1.7), we get the capacity boundary function between bandwidth efficiency C/B and given E b /N 0 : C/B = log C/B E b /N 0 (1.8) Then: E b /N 0 = 2 C/B 1 / C/B (1.9) Figure 1.20 shows the relationship of the capacity boundary of the communication channel with E b /N 0. If the transmission data rate is within the capacity limit, i.e., if R<C,we may be able to achieve transmission rate with properly designed modulation and coding mechanisms, and if R>C, it is impossible to achieve error free transmission. 100 Bandwidth Efficiency (bit/s/hz) Region: R > C 10 Boundary: R = C Region: R < C 1 1.0E E E E E E + 09 Shannon power limit = 0.69 = 1.6 db 0.1 Eb/No Figure 1.20 Capacity boundary of communication channel

36 36 Satellite Networking: Principles and Protocols The Shannon power limit ( 1 6dB) We can increase the bandwidth to reduce transmission power as a trade-off. If we let the transmission bit rate R achieve the maximum, then we can get from Equation (1.8) the following: E b /N 0 1 = log C/B E b /N 0 B/C / Eb/N 0 (1.10) As 1 + 1/x x e when x, let B we can get the Shannon power limit: E b /N 0 = log 2 1/e = log e = 1 6 db (1.11) This tell us, no matter how much bandwidth we have, the transmission power in terms of E b /N 0 should be larger than the Shannon limit, though there is a trade-off between bandwidth and power Shannon bandwidth efficiency for large E b /N 0 Similarly we can derive the formula of Shannon bandwidth efficiency from Equation (1.8) for large E b /N 0, as the following: log 2 C/B E b /N 0 C/B 1 + log 2 C/B E b /N 0 Hence, C/B log 2 E b /N 0, when E b /N 0 Figure 1.21 shows the convergence between C/B and log 2 E b /N 0. It also shows that when transmission power is low, increasing the power by a small amount will have a large impact on the bandwidth efficiency; and when transmission power is high reducing bandwidth efficiency by a small amount will have a large saving on transmission power. Therefore engineers can trade between transmission bandwidth and transmission power, but should not go too far to benefit from such a trade off. 100 Region: R > C Converging Bandwidth Efficiency (bit/s/hz) 10 (C/B) = log 2 (E b /N 0 ) Boundary: R = C Region: R < C 1 1.0E E E E E E E + 18 E b /N 0 Figure 1.21 The Shannon bandwidth efficiency for large E b /N 0

37 Introduction Internetworking with terrestrial networks Internetworking techniques have been well developed in terrestrial networks. When we have different types of networks we face problems at different layers of the protocol stacks, such as different transmission media, different transmission speeds, different data formats and different protocols. Since networking only involves the lower three layers of the protocols, satellite networking with other types of networks could involve any of the three layers Repeaters at the physical layer At the physical layer, internetworking is at bit level. The internetworking repeater needs to have a function to deal with the digital signal. It is relatively easy to internetwork between the terrestrial network and satellite networks, as the physical layer protocol functions are very simple. The main problem is dealing with data transmission rate mismatch, as terrestrial networks may have much higher data transmission rates. The main disadvantage of this solution is that it is inflexible due to the nature of implementation at the physical layer. One may have quickly noticed that the communication payload of transparent satellites, relay satellites or bent-pipe satellites deals with bit streams as functions of a repeater Bridges at link layer A bridge is a store and forward device and is normally used in the context of LANs, interconnecting one or more LANs at the link layer. In satellite networking, we borrow the term to refer to the internetworking unit between the satellite network and terrestrial networks. As it works at the link layer, it also relies on the physical layer transmission, i.e., the bridge deals with the functions of two layers: physical and link layers. A frame arriving from the satellite network will be checked to decide if the frame should be forwarded to the terrestrial networks according to its routing table and the destination address. If yes, the bridge forwards the frame to the terrestrial networks, otherwise it discards it. Before forwarding, the frame is formatted based on the protocol of the terrestrial networks. Similar procedures are also carried out when frames flow from the terrestrial networks to the satellite network. The main disadvantage is that the satellite has to deal with a large number of different types of networks and protocol translations. It has more complicated functions than repeaters. The main advantage is that the satellite network will be able to make use of the link layer functions such as error detection, flow control and frame retransmission. The satellite payload can also implement the bridge functions. Otherwise the link layer functions have to be carried out on the other side of the satellite networks Switches at the physical, link and network layers Switches can work at any layer of the three layers depending on the nature of the networks. Switching networks can set up end-to-end connections to transport bit streams, frames and even network layer packets.

38 38 Satellite Networking: Principles and Protocols The main advantage is that switching networks can reserve network resources when setting up connections. The disadvantages are that they are not very efficient when dealing with short data transmission and supporting connectionless network protocols such as the Internet, and that it is difficult to deal with heterogeneous networks Routers for interconnecting heterogeneous networks Router here refers to an Internet router or an IP router. It deals with only Internet protocol (IP) packets. Figure 1.22 shows how routers can be used to internetwork with heterogeneous terrestrial networks. Here it requires that all user terminals use the IP protocol Protocol translation, stacking and tunnelling It can be seen that there are three basic techniques for interconnecting heterogeneous networks: 1. Protocol translation: this technique is normally used at the physical layer dependent sublayers of the link layer. Protocol translations are carried out between the different sublayers. 2. Protocol stacking: this technique is normally used for different layers. One layer is stacked on top of the other network. 3. Protocol tunnelling: this technique is similar to protocol staking, but with two of the same type of networks communicating through a tunnelling of other networks. Satellite network IP packet T Network A R1 R2 Network B T R3 Router T Network C T User terminal Figure 1.22 Using routers to internetwork with heterogeneous terrestrial networks

39 Introduction Quality of service (QoS) The term quality of service (QoS) is extensively used today. It is not only used in analogue and digital transmission in telephony networks but also in broadband networks, wireless networks, multimedia services and even the Internet. Networks and systems are gradually being designed with consideration of the end-to-end performance required by user applications. Most traditional Internet applications such as and ftp are sensitive to packet loss but can tolerate delays. For multimedia applications (voice and video) this is generally the opposite. They can tolerate some packet loss but are sensitive to delay and variation of the delay. Therefore, networks should have mechanisms for allocating bandwidth resources to guarantee a specific QoS for real-time applications. QoS can be described as a set of parameters that describes the quality of a specific stream of data End-user QoS class and requirements Based on the end-user application requirements, ITU-T recommendation G.1010 defines classification of performance requirements into end-user QoS categories. Based on the target performance requirements, the various applications can be mapped onto axes of packet loss and one-way delay as shown in Figure The size and shape of the boxes provide a general indication of the limit of delay and information loss tolerable for each application class. It can be seen that there are eight distinct groups, which encompass the range of applications identified. Within these eight groupings there is a primary segregation between applications that can tolerate some information loss and those that cannot tolerate any information loss at all, and four general areas of delay tolerance. This mapping is summarised in Figure 1.24, which provides a recommended model for end-user QoS categories, where the four areas of delay are given names chosen to illustrate the type of user interaction involved. Packet Loss 5% 0% Zero loss Conversational voice and video Voice/video messaging Streaming audio/video Delay 100 ms 1 s 10 s Fax 100 s Command /control (e.g. Telnet, Interactive games) Transactions (e.g. E-commerce, Web-browsing, access) Messaging Download (e.g. FTP, still image) Background (e.g. Usenet) Figure 1.23 Mapping of user-centric QoS requirements into network performance (ITUT-G1010) (Reproduced with the kind permission of ITU.)

40 40 Satellite Networking: Principles and Protocols Error tolerant Conversational voice and video Voice/video messaging Streaming audio and video Fax Error intolerant Command/control (e.g. Telnet, interactive games) Transactions (e.g. E-commerce, WWW browsing, access) Messaging, Downloads (e.g. FTP, still image) Background (e.g. Usenet) Interactive (delay <<1 s) Responsive (delay ~2 s) Timely (delay ~10 s) Non-critical (delay >>10 s) Figure 1.24 Model for user-centric QoS categories (ITU-T-G1010) (Reproduced with the kind permission of ITU.) Network performance Network performance (NP) contributes towards QoS as experienced by the user/customer. Network performance may or may not be on an end-to-end basis. For example, access performance is usually separated from the core network performance in the operations of a single IP network, while Internet performance often reflects the combined NP of several autonomous networks. There are four viewpoints of QoS defined by the ITU-T G.1000 recommendation, corresponding with different perspectives, as shown in Figure 1.25: customer QoS requirements; service provider offerings of QoS (or planned/targeted QoS); QoS achieved or delivered; customer survey ratings of QoS. Among these four viewpoints, the customer s QoS requirements may be considered as the logical starting point. A set of customer s QoS requirements may be treated in isolation as far as its capture is concerned. This requirement is an input to the service provider for the determination of the QoS to be offered or planned QoS and NP for satellite networking The definitions of QoS given by the ITU-T are based on a user-centric approach, but these may not reflect well on the QoS and NP related to networking. Therefore it is useful to employ the layering approach to define QoS and NP parameters related to networks (see Figure 1.26).

41 Introduction 41 CUSTOMER Customer's QoS Requirements SERVICE PROVIDER QoS Offered By Provider QoS Perceived By Customer QoS Achieved by Provider Figure 1.25 of ITU.) The four viewpoints of QoS (ITU-T-G1000) (Reproduced with the kind permission User terminals User terminals Terrestrial Network Terrestrial Network Figure 1.26 Network centric Qos & NP User centric Qos & NP User- and network-centric views of QoS and NP concepts The network centric approach enables us to quantify the QoS and NP parameters without the uncertainty of terminal performance, higher layer protocol functions and user factors. Typical parameters are: at analogue transmission level: signal to noise power ratio S/N ; at digital transmission level: bit error ratio BER, propagation delay and delay variation; and at packet level: packet propagation delay and packet delay variation, packet error ratio, packet loss ratio and network throughput.

42 42 Satellite Networking: Principles and Protocols 1.14 Digital video broadcasting (DVB) Digital video broadcasting (DVB) technology allows broadcasting of data containers, in which all kinds of digital data can be transmitted. It simply delivers compressed images, sound or data to the receiver within these containers. No restrictions exist as to the kind of information in the data containers. The DVB service information acts like a header to the container, ensuring that the receiver knows what it needs to decode. A key difference of the DVB approach compared to other data broadcasting systems is that the different data elements within the container can carry independent timing information. This allows, for example, audio information to be synchronised with video information in the receiver, even if the video and audio information does not arrive at the receiver at exactly the same time. This facility is, of course, essential for the transmission of conventional television programmes. The DVB approach provides a good deal of flexibility. For example, a 38 Mbit/s data container could hold eight standard definition television (SDTV) programmes, four enhanced definition television (EDTV) programmes or one high definition television (HDTV) programme, all with associated multi-channel audio and ancillary data services. Alternatively, a mix of SDTV and EDTV programmes could be provided or even multimedia data containing little or no video information. The content of the container can be modified to reflect changes in the service offer over time (e.g. migration to a widescreen presentation format). At present, the majority of DVB satellite transmissions convey multiple SDTV programmes and associated audio and data. DVB is also useful for data broadcasting services (e.g. access to the World Wide Web) The DVB standards Digital video broadcasting (DVB) is a term that is generally used to describe digital television and data broadcasting services that comply with the DVB standard. In fact, there is no single DVB standard, but rather a collection of standards, technical recommendations and guidelines. These were developed by the Project on Digital Video Broadcasting, usually referred to as the DVB Project. The DVB Project was initiated in 1993 in liaison with the European Broadcasting Union (EBU), the European Telecommunications Standards Institute (ETSI) and the European Committee for Electrotechnical Standardisation (CENELEC). The DVB Project is a consortium of some 300 member organisations. As opposed to traditional governmental agency standards activities round the world, the DVB Project is market-driven and consequently works on commercial terms, to tight deadlines and realistic requirements, always with an eye toward promoting its technologies through achieving economies of scale. Though based in Europe, the DVB Project is international, and its members are in 57 countries round the globe. DVB specifications concern: source coding of audio, data and video signals; channel coding; transmitting DVB signals over terrestrial and satellite communications paths; scrambling and conditional access;

43 Introduction 43 the general aspects of digital broadcasting; software platforms in user terminals; user interfaces supporting access to DVB services; the return channel, as from a user back to an information or programme source to support interactive services. The DVB specifications are interrelated with other recognised specifications. DVB source coding of audio-visual information as well as multiplexing is based on the standards evolved by the Moving Picture Experts Group (MPEG), a joint effort of the International Organisation for Standards (ISO) and the International Electrotechnical Commission (IEC). The principal advantage of MPEG compared to other audio and audio coding formats is that the sophisticated compression techniques used make MPEG files far smaller for the same quality. For instance, the first standard, MPEG1, was introduced in 1991 and supports 52:1 compression, while the more recent MPEG2 supports compression of up to 200:1. The DVB Project is run on a voluntary basis and brings together experts from more than 300 companies and organisations, representing the interests of manufacturing industries, broadcasters and services providers, network and satellite operators and regulatory bodies. Its main intent is to reap the benefits of technical standardisation, while at the same time satisfying the commercial requirements of the project members. Although a large part of the standardisation work is now complete, work is still ongoing on issues such as the Multimedia Home Platform. Much of the output of the DVB Project has been formalised by ETSI DVB-S satellite delivery One of the earliest standards developed by the DVB Project and formulated by ETSI was for digital video broadcasting via satellite (usually referred to as the DVB-S standard ). Specifications also exist for the retransmission of DVB signals via cable networks and satellite master antenna television (SMATV) distribution networks. The techniques used for DVB via satellite are classical in the sense that they have been used for many years to provide point-to-point and point-to-multipoint satellite data links in professional applications. The key contribution of the DVB Project in this respect has been the development of highly integrated and low-cost chip sets that adapt the DVB baseband signal to the satellite channel. Data transmissions via satellite are very robust, offering a maximum bit error rate in the order of In satellite applications, the maximum data rate for a data container is typically about 38 Mbit/s. This container can be accommodated in a single 33 MHz satellite transponder. It provides sufficient capacity to deliver, for example, four to eight standard television programmes, 150 radio channels, 550 ISDN channels, or any combination of these services. This represents a significant improvement over conventional analogue satellite transmission, where the same transponder is typically used to accommodate a single television programme with far less operational flexibility. A single modern high-power broadcasting satellite typically provides at least twenty 33 MHz transponders, allowing delivery of about 760 Mbit/s of data to large numbers of users equipped with small (around 60 cm) satellite dishes. A simple generic model of a digital satellite transmission channel comprises several basic building blocks, which include baseband processing and channel adaptation in the transmitter

44 44 Satellite Networking: Principles and Protocols and the complementary functions in the receiver. Central to the model is, of course, the satellite transmission channel. Channel adaptation would most likely be done at the transmit satellite earth station, while the baseband processing would be performed at a point close to the programme source MPEG-2 baseband processing MPEG is a group of experts drawn from industry who contribute to the development of common standards through an ITU-T and ISO/IEC joint committee. The established MPEG- 2 standard was adopted in DVB for the source coding of audio and video information and for multiplexing a number of source data streams and ancillary information into a single data stream suitable for transmission. Therefore, many of the parameters, fields and syntax used in DVB baseband processing are specified in the relevant MPEG-2 standards. The MPEG-2 standards are generic and very wide in scope. Some of the parameters and fields of MPEG-2 are not used in DVB. The processing function deals with a number of programme sources. Each programme source comprises any mixture of raw data and uncompressed video and audio, where the data can be, for example, teletext and/or subtitling information and graphical information such as logos. Each of the video, audio and programme-related data is called an elementary stream (ES). It is encoded and formatted into a packetised elementary stream (PES). Thus each PES is a digitally encoded component of a programme. The simplest type of service is a radio programme, which would consist of a single audio elementary stream. A traditional television broadcast would comprise three elementary streams: one carrying coded video, one carrying coded stereo audio and one carrying teletext Transport stream (TS) Following packetisation, the various elementary streams of a programme are multiplexed with packetised elementary streams from other programmes to form a transport stream (TS). Each of the packetised elementary streams can carry timing information, or time stamps, to ensure that related elementary streams, for example, video and audio, are replayed in synchronism in the decoder. Programmes can each have a different reference clock, or can share a common clock. Samples of each programme clock, called programme clock references (PCRs), are inserted into the transport stream to enable the decoder to synchronise its clock to that in the multiplexer. Once synchronised, the decoder can correctly interpret the time stamps and can determine the appropriate time to decode and present the associated information to the user. Additional data is inserted into the transport stream, which includes programme specific information (PSI), service information (SI), conditional access (CA) data and private data. Private data is a data stream whose content is not specified by MPEG. The transport stream is a single data stream that is suitable for transmission or storage. It may be of fixed or variable data rate and may contain fixed or variable data rate elementary streams. There is no form of error protection within the multiplex. Error protection is implemented within the satellite channel adaptor.

45 Introduction Service objectives The DVB-S system is designed to provide so-called quasi error free (QEF) quality. This means less than one uncorrected error event per transmission hour, corresponding to a bit error rate (BER) of between and at the input of the MPEG-2 demultiplexer (i.e. after all error correction decoding). This quality is necessary to ensure that the MPEG-2 decoders can reliably reconstruct the video and audio information. This quality target translates to a minimum carrier-to-noise ratio C/N requirement for the satellite link, which in turn determines the requirements for the transmit earth station and the user s satellite reception equipment for a given satellite broadcasting network. The requirement is actually expressed in E b /N 0 (energy per bit to noise density ratio), rather than C/N, so that it is independent of the transmission rate. The DVB-S standard specifies the E b /N 0 values at which QEF quality must be achieved when the output of the modulator is directly connected to the input of the demodulator (i.e. in an IF loop ). An allowance is made for practical implementation of the modulator and demodulator functions and for the small degradation introduced by the satellite channel. The values range from 4.5 db for rate 1/2 convolutional coding to 6.4 db for rate 7/8 convolutional coding. The inner code rate can be varied to increase or decrease the degree of error protection for the satellite link at the expense of capacity. The reduction or increase in capacity associated with a change in the code rate and the related increase or reduction in the E b /N 0 requirement. The latter is also expressed as an equivalent increase or reduction in the diameter of the receive antenna (the size of user s satellite dish), all other link parameters remaining unchanged Satellite channel adaptation The DVB-S standard is intended for direct-to-home (DTH) services to consumer integrated receiver decoders (IRD), as well as for reception via collective antenna systems (satellite master antenna television (SMATV)) and at cable television head-end stations. It can support the use of different satellite transponder bandwidths, although a bandwidth of 33 MHz is commonly used. All service components ( programmes ) are time division multiplexed (TDM) into a single MPEG-2 transport stream, which is then transmitted on a single digital carrier. The modulation is classical quadrature phase shift keying (QPSK). A concatenated error protection strategy is employed based on a convolutional inner code and a shortened Reed Solomon (RS) outer code. Flexibility is provided so that transmission capacity can be traded off against increased error protection by varying the rate of the convolutional code. Satellite links can therefore be made more robust, at the expense of reduced throughput per satellite transponder (i.e. fewer DVB services). The standard specifies the characteristics of the digitally modulated signal to ensure compatibility between equipment developed by different manufacturers. The processing at the receiver is, to a certain extent, left open to allow manufacturers to develop their own proprietary solutions. It also defines service quality targets and identifies the global performance requirements and features of the system that are necessary to meet these targets.

46 46 Satellite Networking: Principles and Protocols DVB return channel over satellite (DVB-RCS) The principal elements of a DVB return channel over satellite (DVB-RCS) system are the hub station and user satellite terminals. The hub station controls the terminals over the forward (also called outbound link), and the terminals share the return (also called inbound link). The hub station continuously transmits the forward link in time division multiplex (TDM). The terminals transmit as needed, sharing the return channel resources using multi-frequency time division multiple access (MF-TDMA). The DVB-RCS system supports communications on channels in two directions: Forward channel, from the hub station to many terminals. Return channels, from the terminals to the hub station. The forward channel is said to provide point-to-multipoint service, because it is sent by a station at a single point to stations at many different points. It is identical to a DVB-S broadcast channel and has a single carrier, which may take up the entire bandwidth of a transponder (bandwidth-limited) or use the available transponder power (power limited). Communications to the terminals share the channel by using different slots in the TDM carrier. The terminals share the return channel capacity of one or more satellite transponders by transmitting in bursts, using MF-TDMA. In a system, this means that there is a set of return channel carrier frequencies, each of which is divided into time slots which can be assigned to terminals, which permits many terminals to transmit simultaneously to the hub. The return channel can serve many purposes and consequently offers choices of some channel parameters. A terminal can change frequency, bit rate, FEC rate, burst length, or all of these parameters, from burst to burst. Slots in the return channel are dynamically allocated. The uplink and downlink transmission times between the hub and the satellite are very nearly fixed. However, the terminals are at different points, so the signal transit times between them and the satellite vary. On the forward channel, this variation is unimportant. Just as satellite TV sets successfully receive signals whenever they arrive, the terminals receive downlink signals without regard to small differences in their times of arrival. However, on the uplink, in the return direction from the terminals to the hub, small differences in transit time can disrupt transmission. This is because the terminals transmit in bursts that share a common return channel by being spaced from each other in time. For instance, a burst from one terminal might be late because it takes longer to reach the satellite than a burst sent by another terminal. A burst that is earlier or later than it should be can collide with the bursts sent by the terminals using the neighbouring TDMA slots. The difference in transmission times to terminals throughout the footprint of a satellite might be compensated for by using time slots that are considerably longer than the bursts transmitted by the terminals, so both before and after a burst there is a guard time sufficiently long to prevent collisions with the bursts in neighbouring slots in the TDMA frame. The one-way delay time between a hub and a terminal varies from 250 to 290 ms, depending on the geographical location of the terminal with respect to the hub. So the time differential, T, might be as large as 40 ms. So most TDMA satellite systems minimise guard time by incorporating various means of timing adjustment to compensate for satellite path differences.

47 Introduction 47 DVB-RCS has two built-in methods of pre-compensating the burst transmission time of each terminal: Each terminal knows its local GPS coordinates and therefore can calculate its own burst transmission time. The hub monitors the arrival times of bursts, and can send correction data to terminals if need be TCP/IP over DVB DVB-RCS uses the MPEG-2 digital wrappers, in which protocol-independent client traffic is enclosed within the payloads of a stream of 188-byte packets. The MPEG-2 digital wrapper offers a 182-byte payload and has a 6-byte header. The sequence for transmission of Internet TCP/IP traffic includes: The TCP/IP message arrives and is subjected to TCP optimisation. The IP packets are divided into smaller pieces and put into data sections with 96-bit digital storage medium command and control (DSM-CC) headers. The DSM-CC data sections are further divided into 188-byte MPEG2-TS packets in the baseband processing. The MPEG2-TS packets then are subjected to channel coding for satellite transmissions Historical development of computer and data networks Telecommunication systems and broadcasting systems have been developing for over 100 years. The basic principles and services have changed little since their beginnings and we can still recognise the earliest telephony systems and televisions. However, computers and the Internet have changed greatly in the last 40 years. Today s systems and terminals are completely different from those used 40 or even 10 years ago. The following gives a quick review of these developments to show the pace of technology progress The dawn of the computer and data communications age The first electronic digital computer was developed during Early computer interfaces used punched tapes and cards. Later terminals were developed and the first communication between terminals and computer over long distances was in 1950, which used voice-grade telephone links at low transmission speeds of 300 to 1200 kbit/s. Automatic repeat requests (ARQ) for error correction were mainly used for data transmission Development of local area networks (LANs) From 1950 to 1970 research carried out on computer networks led to the development of different types of network technologies local area networks (LANs), metropolitan area networks (MANs) and wide area networks (WANs).

48 48 Satellite Networking: Principles and Protocols A collection of standards, known as IEEE 802, was developed in the 1980s including the Ethernet as IEEE802.3, token bus as IEEE802.4, token ring as IEEE802.5, DQDB as IEEE802.6 and others. The initial aim was to share file systems and expensive peripheral devices such as high-quality printers and graphical plot machines at fast data rates Development of WANs and ISO/OSI The ISO developed the Open System Interconnection (OSI) reference model with seven layers for use in wide area networks in the 1980s. The goal of the reference model was to provide an open standard so that different terminals and computer systems could be connected together if they conformed to the standard. The terminals considered in the reference model were connected to a mainframe computer over a WAN in text mode and at slow speed The birth of the Internet Many different network technologies were developed during the 1970s and 1980s and many of them did not fully conform to international standards. Internetworking between different types of networks used protocol translators and interworking units, and became more and more complicated as the protocol translators and interworking units became more technology dependent. In the 1970s, the Advanced Research Project Agency Network (ARPARNET) sponsored by the US Department of Defense developed a new protocol, which was independent of network technologies, to interconnect different types of networks. The ARPARNET was renamed as the Internet in The main application layer protocols included remote telnet for terminal access, FTP for file transfer and for sending mail through computer networks Integration of telephony and data networks In the 1970s, the ITU-T started to develop standards called integrated services digital networks with end-to-end digital connectivity to support a wide range of services, including voice and non-voice services. User access to the ISDN was through a limited set of standard multipurpose customer interfaces. Before ISDN, access networks, also called local loops, to the telecommunication networks were analogue, although the trunk networks, also called transit networks, were digital. This was the first attempt to integrate telephony and data networks and integration of services over a single type of network. It still followed the fundamental concepts of channel- and circuit-based networks used in traditional telecommunication networks Development of broadband integrated networks As soon as the ISDN was completed in the 1980s, the ITU-T started to develop broadband ISDN. In addition to broadband integrated services, ATM technology was developed to support the services based on fast packet-switching technologies. New concepts of virtual

49 Introduction 49 channels and circuits were developed. The network is connection oriented, which allows negotiation of bandwidth resources and applications. It was expected to unify the telephony networks and data networks and also unify LANs, MANs and WANs. From the LAN aspect, ATM faced fierce competition from fast Ethernet. From application aspects, it faced competition from the Internet The killer application WWW and Internet evolutions In 1990, Tim Berners-Lee developed a new application called the World Wide Web (WWW) based on hypertext over the Internet. This significantly changed the direction of network research and development. A large number of issues needed to be addressed to cope with the requirements of new services and applications, including real-time services and their quality of service (QoS), which were not considered in traditional Internet applications Historical development of satellite communications Satellite has been associated with telecommunications and television from its beginning, but few people have noticed this. Today, satellites broadcast television programmes directly to our homes and allow us to transmit messages and surf the Internet. The following gives a quick review of satellite history Start of satellite and space eras Satellite technology has advanced significantly since the launch of the first artificial satellite Sputnik by the USSR on 4 October 1957 and the first experiment of an active relaying communications satellite Courier-1B by the USA in August The first international cooperation to explore satellite for television and multiplexed telephony services was marked by the experimental pre-operation transatlantic communications between the USA, France, Germany and the UK in Early satellite communications: TV and telephony Establishment of the Intelsat organisation started with 19 national administration and initial signatories in August The launch of the REARLY BIRD (Intelsat-1) marked the first commercial geostationary communication satellite. It provided 240 telephone circuits and one TV channel between the USA, France, Germany and the UK in April In 1967, Intelsat-II satellites provided the same service over the Atlantic and Pacific Ocean regions. From 1968 to 1970, Intelsat-III achieved worldwide operation with 1500 telephone circuits and four TV channels. The first Intelsat-IV satellite provided 4000 telephone circuits and two TV channels in January 1971 and Intelsat-IVa provided 20 transponders of 6000 circuits and two TV channels, which used beam separation for frequency reuse.

50 50 Satellite Networking: Principles and Protocols Development of satellite digital transmission In 1981, the first Intelsat-V satellite achieved capacity of circuits with FDMA and TDMA operations, 6/4 GHz and 14/11 GHz wideband transponders, and frequency reuse by beam separation and dual polarisation. In 1989, the Intelsat-VI satellite provided onboard satellite-switched TDMA of up to circuits. In 1998, Intelsat VII, VIIa and Intelsat- VIII satellites were launched. In 2000, the Intelsat-IX satellite achieved circuits Development of direct-to-home (DTH) broadcast In 1999, the first K-TV satellite provided 30 14/11-12 GHz transponders for 210 TV programmes with possible direct-to-home (DTH) broadcast and VSAT services Development of satellite maritime communications In June 1979, the International Maritime Satellite (Inmarsat) organisation was established to provide global maritime satellite communication with 26 initial signatories. It explored the mobility feature of satellite communications Satellite communications in regions and countries At a regional level, the European Telecommunication Satellite (Eutelsat) organisation was established with 17 administrations as initial signatories in June Many countries also developed their own domestic satellite communications systems, including the USA, the USSR, Canada, France, Germany, the UK, Japan, China and other nations Satellite broadband networks and mobile networks Since the 1990s, significant development had been carried out on broadband networks including onboard-switching satellite technologies. Various non-geostationary satellites have been developed for mobile satellite services (MSSs) and broadband fixed satellite services (FSSs) Internet over satellite networks Since the late 1990s and the start of the twenty-first century, we have seen a dramatic increase in Internet traffic over the communication networks. Satellite networks have been used to transport Internet traffic in addition to telephony and television traffic for access and transit networks. This brings great opportunities as well as challenges to the satellite industry. On one hand, it needs to develop internetworking with many different types of legacy networks; and on the other hand, it needs to develop new technologies to internetwork with future networks. We have also see the convergence of different types of networks including network technologies, network protocols and new services and applications.

51 Introduction Convergence of network technologies and protocols The convergence is the natural progression of technologies pushing and user demands pulling and the development of business cases. Obviously, satellite networking closely follows the development of terrestrial networks, but is capable of overcoming geographical barriers and the difficulty of wide coverage faced by terrestrial networks. Figure 1.27 illustrates the vision of a future satellite network in the context of the global information infrastructure Convergence of services and applications in user terminals In the early days, user terminals were designed for particular types of services and had very limited functions. For example, we had telephone handsets for voice services, computer terminals for data services, and television for receiving television services. Different networks were developed to support these different types of terminals. As the technology developed, additional terminals and services were introduced into the existing networks. For example, fax and computer dialup services were added to telephone networks. However, the transmission speeds of fax and dialup links were limited by the capacity of the telephone channel supported by the telephony networks. Computer terminals have become more and more sophisticated and are now capable of dealing with voice and video services in real time. Naturally, in addition to data services, there are increasing demands to support real time voice and video over data networks. Multimedia services, a combination of voice, video and data, were developed. These complicate the QoS requirements requiring complicated user terminal and network design, implementation and operation. To support such services over satellite networks for applications such as aeronautics, shipping, transport and emergency services brings even more challenges. We are starting to see the convergence of different user terminals for different types of services into a single user terminal for all types of services. Satellite Global Suburban Urban In-Building In-Home Macro-cell Micro-cell Pico-cell Home-cell dik Figure 1.27 Satellite in the global information infrastructure